In commonly owned U.S. Pat. No. 4,017,236 there has been disclosed an injection-molding machine of this type which has a piston head hydraulically reciprocated within a cylinder and a piston rod secured to a tubular thrust member or column axially surrounding that rod, the thrust member being rigid with the movable platen which is guided by the usual tie bars. When this platen is advanced by the hydraulically operated driving piston or plunger into a forward position in which the mold is nearly closed, a pair of force-transmitting elements or shutters move in behind the column and are in turn pressurized by a cascade of ancillary pistons received in annular cylinder chambers of a stationary housing. The patent also describes a so-called mold-breaking action, i.e. an initial separation of the mold plates carried by the platens after the injection of thermoplastic material into the mold, involving the admission of hydraulic liquid to an annular face on a peripheral formation or head of the main cylinder which for this purpose is limitedly axially shiftable in the housing.
With molding machines having a long stroke, the axial mobility of the correspondingly lengthened main cylinder may give rise to problems in maintaining that cylinder precisely parallel to the tie bars.
Another problem sometimes encountered in a molding machine of this description is the danger of jamming in the event of failure of the supply of hydraulic liquid (referred to hereinafter, for convenience, as oil) to the annular housing chamber of chambers containing the ancillary piston or pistons of the mold-clamping mechanism. If these pistons are not properly pressurized to complete the closure of the mold, the injected plastic material may drive the mold plates apart and wedge the shutters in their operating position between the column and the housing. If the ancillary pistons make metal-to-metal contact with the rear walls of their chambers in their retracted position, the shutters could not be released by oil pressure and could be liberated only by destructive measures such as cutting off the retaining nuts of the tie bars.